Solar cells are photovoltaic devices that use semiconductors to convert photons into electrical energy. In a semiconductor, a conduction band and a valence band are separated by an energy gap Eg that varies with material composition and temperature. When a photon is absorbed by a semiconductor, an electron is promoted from the valence band into the conduction band, thereby creating a hole in the valence band. A photon of wavelength λ and frequency ν has an energy hν=hc/λ and is generally absorbed by a semiconductor when hν≧Eg. However, any extra energy in the photon is converted into thermal rather than electrical energy, since only one electron-hole pair can be created for each absorption event. On the other hand, a semiconductor is more transparent to wavelengths corresponding to energies less than Eg, since in this case the photons are not energetic enough to promote electrons from the valence band into the conduction band. Thus, a single band gap-based system cannot exceed 32% conversion efficiency for untreated sunlight, since the most energetic photons produce largely thermal energy and are therefore inefficiently utilized, while the least energetic photons cannot be absorbed.
One technique for increasing the overall conversion efficiency is to use multiple cells with different band gaps to convert different parts of the illuminating solar spectrum, with each cell optimized for the restricted illuminating spectrum that it receives. This configuration called tandem cell configuration is a series of several cells each of which is optimized to a part of the solar spectrum, positioned one on top of the other, in a decreasing band gap order. The cells that make up the tandem can be grown individually and stacked together in a mechanical fashion, or the entire device may be grown monolithically using any of the known growth techniques (for example metal-organic chemical vapour deposition (MOCVD), molecular beam epitaxy (MBE), and liquid-phase epitaxy (LPE). Each cell in a mechanical stack requires its own substrate for growth, which increases the overall cost. Additionally, complex to engineering is required to provide good electrical connection to the stack, good thermal connections between the cells to dissipate heat which would otherwise reduce efficiency, and good optical connection between the cells. Overall, such cells tend to suffer from poor efficiency and poor reliability. For these reasons, monolithic stacks in which the cells are grown one on another on a common substrate are preferred. In a monolithic cell structure there is a requirement to create an ohmic electrical connection between the different band gap regions. This is achieved by the use of tunnel diodes between the cells so that the overall structure has only two electrical connections. The individual cells within the structure are connected in series so that the current through any cell is the same for all cells. This design leads to a current constraint whereby each cell must generate the same current for efficient operation. It is possible to design and optimize a structure for a particular spectrum, but when used in practice, such as in a terrestrial solar concentrator system, the spectrum will change throughout the day and throughout the year. This means that for much of the time the individual cells will not be current matched and the device efficiency will be reduced from the optimum value recorded when under the designed illumination spectrum. Furthermore, temperature variation is significant in a concentrator system so that the cell band gap variation will mean that the efficiency is reduced from the current matched optimum.
An alternative technique for increasing the overall conversion efficiency is to achieve the required spectral splitting using optics to deflect the correct part of the spectrum to the relevant cell. Spectral splitting consists of optical splitting of the incident illumination into several windows of energy ranges, each of which is suitable for cells that are optimized to the specific energy range. Unfortunately, this approach suffers from the difficulty in splitting the incident illumination over large areas because of the diffuse nature of the sunlight. Therefore, radiation concentrators are typically required.
Another technique for increasing the overall conversion efficiency is to use up- and down-conversion to convert two low energy photons to one of high energy, and split one high energy photon into two low energy ones, respectively, to make better use of the solar spectrum for a given bandgap system. However, even efficient use of both up-and down conversion, leads to only modest increases in solar-to-electrical energy conversion efficiency.
Another shortcoming of such solar cells, mainly amorphous silicon and molecular semiconductor (MSC) based systems, is the high internal resistivity of the photoactive materials. The process of photovoltaic energy conversion in molecular solar cells can be divided into three steps; creation of a movable electron/hole pair called an exciton in the MSC by light absorption, charge separation and charge collection at the two metal contacts. Usually charge separation occurs at the interface between the MSC and a hole (or electron) selective material that dissociates (quenches) excitons across the interface. To complete the circuit, the charges created at the MSC-quencher interface must be collected selectively at the metal contacts. Since the charge generated at the MSC-quencher interface has to travel across the MSC layer, the collection efficiency depends on the electrical resistivity of the MSCs. In other words, efficient charge separation requires long exciton diffusion length and high quality of the quencher-USC (Organic Semiconductor) interface, while efficient light conversion depends also on the electrical resistivity of the MSC layer. Moreover either short exciton diffusion length or high resistivity of the MSCs limits the thickness of the MSC layers in which the high quantum yield of the energy conversion process is maintained. Consequently, the overall optical density and thus the conversion efficiency of these solar cells are low.
Interpenetrating configurations, offering high interface area per illuminated area solve the exciton diffusion length problem, but often complicate the transport of the photo-generated charges.